The invention concerns a magnetic shielding for a superconducting magnetic coil arranged on the inside of a cryostat and having a vertical axis for the production of a static magnetic field with a homogeneity of &lt;10.sup.-7 in an investigational volume of a nuclear magnetic resonance (NMR) spectrometer.
A magnetic shielding of this kind is known in the art from the company brochure "Introducing new technology for Very High Field NMR Spectrometers", Houston Advanced Research Center, 4800 Research Forest Drive, The Woodlands, Tex. 77381, USA, 1993.
Although different types of magnetic shields exist in the area of NMR tomography, room shieldings are not at all normally used in the analytic area of NMR. Sometimes one or several iron plates are arranged at a large distance from the room in which the relevant NMR analytic magnet is located, normally in the upper and lower floors relative to this room and effect a very incomplete and relatively ineffective shielding.
Completely enclosed or symmetric shielding chambers are known in the art of NMR tomography, for example, through the article "A Cylindrically Symmetric Magnetic Shield for a Large-Bore 3.0 Tesla Magnet", by Ewing et al., MRM 29, pp. 394-401, 1993. The NMR magnetic coil lies, with these types of closed and symmetric tomography shields, at the symmetry center of the entire configuration. For geometric reasons it is not possible to symmetrically arrange a room shielding for NMR analytic magnets, since the analytic magnet cryostat requires substantially more room in the upward direction than towards the floor of the room. If one then, with normal room sizes (for example 3 m ceiling height for an analytic laboratory), attempts to construct a magnetic room shielding through the use of iron plates introduced in the ceiling and in the floor, one would, in order to arrange the NMR analytic magnet symmetrically with respect to the ceiling and the floor, either have to completely reconstruct the cryostat or eliminate the conventional legs of the cryostat, whereby access to the NMR apparatus from below would no longer be possible. With normal cryostats a symmetric shielding solution is only conceivable with particularly high rooms (approx. 4 m ceiling height) or with the use of an iron plate configured approximately 1 m above the flooring of the next upper floor which would be extremely difficult and not economically sensible.
Since the homogeneity of the static magnetic field produced in NMR analytic applications (.ltoreq.10.sup.-7) must exceed that of tomography magnet systems (approx. 10.sup.-4 to 10.sup.-5) for an NMR analytic magnet, a simple asymmetric room shielding configuration is not usable. Furthermore, such an asymmetric configuration would cause enormous forces on the shielding elements so that, for this reason alone, an asymmetric room temperature shielding for NMR analytic magnets constructed with simple ferromagnetic plates on the floor and on the ceiling of the room being shielded can be ruled out.
Another conceivable possibility would be a symmetric shielding cooled in the helium bath of the cryomagnet. Towards this end it would, however, be necessary to substantially modify the magnet system as well as the cryostat compared to the usual conventional devices so that existing magnet or cryostat designs could not be utilized.
It is therefore the purpose of the present invention to create a magnetic shielding for an NMR analytic magnet of the above mentioned kind which can be realized for conventional existing magnet coils and cryostat systems without major modification, whereby neither increased force build-up problems nor significant distortion of the homogeneity of the produced magnetic field occur.